The invention relates to a method for passivating a metallic surface of a lightweight metal component and a passivation solution for forming a conversion layer for the metallic surface of the lightweight metal component.
The use of lightweight metal components is common practice across industries. Especially in vehicle construction for example vehicle bodies can be manufactured as composite construction for example from lightweight metal and steel parts in order to reduce weight. The lightweight metals can for example be aluminum or magnesium materials or alloys thereof.
From DE 196 30 289 C2 a generic method for painting vehicle bodies is known. According to this the body-in-white is subjected to a pre-treatment prior to the actual painting process in which pre-treatment vehicle bodies are first degreased in a spray- and full submersion zone. Subsequently the cleaned vehicle bodies are coated with a zinc-phosphate layer in a phosphate bath. This layer serves as additional corrosion protection and as a bonding agent for the subsequent primer application. Subsequent to this pre-treatment step a cataphoresis-priming is performed in the dip method in which an electric direct voltage is applied and the paint particles that are dissolved in the dip bath are attracted by the sheet metal of the vehicle body where they adhere to form a primer coat. Subsequently the primed vehicle body is transported into a downstream continuous furnace in which the primer coat is burned in. Subsequent thereto the vehicle body that is provided with the primer coat is transported to a further coating station in which a top coat in the color requested by the customer is applied. Hereby the paint particles can be transported through an electrostatic field from support heads that are under high voltage to the grounded vehicle body. Downstream of this top coating process also a continuous furnace is arranged in which temperature a the topcoat is cured at high temperature. Subsequently a clear varnish is applied in a further coating station, which clear varnish is cured at high temperature.
In vehicle bodies produced with a composite construction method the lightweight metal components (made of Mg or Al) are more corrosion sensitive compared to the steel components. In particular paint creeps and filiform corrosion are frequent causes for damage. In order to protect the lightweight metal components it is common to use a pickling passivation and to apply an anodic coating. The commercial coatings however only offer limited required protection against self-corrosion, filiform corrosion and/or paint creep. In particular in contact with magnesium the high potential difference promotes corrosion.
The commercially available coating systems for lightweight components do not have sufficient passivating properties and are oftentimes too “noble” relative to magnesium (i.e., excessive corrosion potentials). When magnesium alloys are electrochemically polarized with more noble metals (for example aluminum) the corrosion current increases exponentially.